The invention relates to radio frequency communication systems and methods and more particularly to a system and method which provides for the dynamic adjustment of information density communicated.
Information communication between systems separated by significant physical distances has been an obstacle to integration of such systems. The choices available to bridge the physical gap between such systems have not only been limited, but have required undesirable tradeoffs in cost, performance, and reliability.
For example microwave radio links may be deployed to bridge such gaps. However, microwave, as well as other wireless, communications experience signal degradation due to link characteristics which may include both constant and time varying factors. For example, link signals experience attenuation as a function of the link distance which is a constant. Additionally, signal attenuation may be time variant as a function of atmospheric conditions, such as the presence of precipitation, i.e., rain, within the link. Moreover, these atmospheric conditions may not be constant throughout the link and, therefore, may affect the link signal differently at different portions of the link.
Presently, microwave radio links are generally designed or engineered with, often substantial, fade margins to guarantee high signal availability even in the presence of time varying fades, such as may be caused by rainfall. For example, it is often desired that a link have an availability of 99.995%, even in the presence of deep fades associated with the above described rainfall. Accordingly, link communications parameters may be adjusted at deployment to provide this level of availability.
The data rate at which a link may operate is generally a function of the minimum signal to noise ratio present during the worst case fade, i.e., in the above mentioned case the 99.995% point. Accordingly, the link parameters which are adjusted to provide the desired link availability may include the link transmit power level and/or link information density, i.e., increasing the transmit power level to a magnitude sufficient for the deepest expected fade and/or reducing the information density to a point where the maximum fade may be tolerated without an unacceptable error rate.
However, the worst case conditions may only manifest occasionally. Accordingly, the bulk of the time the link may operate at a much higher signal to noise ratio than is necessary to provide the desired communications. Accordingly, a need exists in the art for a system and method for dynamically adjusting link information density to exploit the capability of the link supporting a much higher data rate during times of less than maximum expected attenuation experienced in the link.
A further need exists in the art for the system and method for adjusting link information density to operate at an optimum data rate which does not sacrifice a desired level of link availability.
A still further need exists in the art for the adjusting of link information density to be independent with respect to any system communicating via a common link in order to allow for independent optimization of the information density for each such system.
These and other objects, features and technical advantages are achieved by a system and method which monitors link conditions and dynamically selects a particular modulation level from a multi-level modulation format, such as qadrature amplitude modulation (QAM), phase shift keying (PSK), frequency shift keying (FSK), amplitude shift keying (ASK),carrier less amplitude phase (CAP), or the like, determined to be optimum for the presently experienced link conditions. Preferably, the link conditions are monitored in both the uplink and downlink in order to achieve the desired level of availability in both the forward and reverse channels of a link. Moreover, uplink and downlink monitoring of parameters allows the present invention to provide link optimization independently in both the forward and reverse channels.
According to a preferred embodiment of the present invention, a constant baud rate but variable index (level) modulation format, such as QAM, is utilized in a radio link having a payload field and control field. Link parameters, such as a received signal strength indicator (RSSI) and/or a signal to noise ratio (SNR or signal to noise ratio estimate (SNRE), are continuously monitored at a receiver. The modulation index used is a function of the RSSI and/or SNRE and is dynamically changed by way of signaling over the aforementioned control field.
Accordingly, during periods of low RSSI and/or SNRE, such as those typically present during the deepest rain fades, the link will operate at a minimum modulation index, such as 4-QAM. As the RSSI and/or SNRE increase, i.e., the rain subsides, the modulation indexes, and thus the data rate, of the link is increased. For example, the link could be engineered for 99.995% availability while delivering a data rate X in 4-QAM mode. However, during periods of high signal to noise the link would operate at a higher modulation index, such as 256-QAM delivering data at a rate of 4X. Intermediate modulation indexes, such as 16-QAM, having a data rate of 2X, and 64-QAM, having a data rate of 3X, may also be utilized based on monitored link parameters.
A preferred embodiment of the present invention utilizes time division multiple access (TDMA) bursts and frames in order to provide seemingly simultaneous communications associated with a plurality of systems. Each of these systems may be disposed in a somewhat different location in the link. Accordingly, ones of these systems may have a different link distance, having associated therewith a constant link signal attenuation, and therefore may have a different minimum and/or maximum modulation index corresponding to the link distance.
Time varying link conditions may not be consistent throughout the link. Moreover, such conditions may be cumulative such that the longer the link the greater the affect associated with the condition. For example, a system located such that a relatively short air gap is used will experience less link signal attenuation due to rain than a system located such that a relatively long air gap is used, even when the rain density is consistent throughout the area of the link.
Accordingly, a preferred embodiment of the present invention provides for the adjustment of information density independently for frames and/or bursts associated with ones of the communication systems. For example, a first burst associated with a system utilizing the aforementioned relatively long link may include payload modulated in 4-QAM mode during a rain event while a second burst associated with a system utilizing the aforementioned relatively short link may include payload modulated in 64-QAM mode.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.